Recently, thorough research, development and businesses for producing bioethanol fuel are being introduced. However, the major problem of bioethanol which is a gasoline blending stock is that, when water is introduced to ethanol-mixed gasoline, it is dissolved into the mixed gasoline, thus water-ethanol mixture is separated from gasoline.
Compared to such ethanol, butanol-mixed gasoline does not absorb water even when water is introduced thereto, and thus, separation of butanol does not occur. Accordingly, the butanol-mixed gasoline does not require the use of additional devices in its storage, transport, supply systems and in vehicles in which it can be used, unlike ethanol-mixed gasoline.
In addition, butanol is advantageous because it has lower vapor pressure than ethanol, thus reducing a probability of causing vapor lock in an automobile engine. Also, unlike ethanol, butanol has an air to fuel ratio similar to that of gasoline, which implies a relatively larger amount of butanol may be mixed with gasoline in a range that does not affect engine performance.
As is apparent from Table 1 below, however, butanol is disadvantageous because it has an octane number approximately equal to that of gasoline, and thus is difficult to use as an octane number booster such as ethanol, MTBE or ETBE.
TABLE 1EthanolButanolGasolineSolubility in WaterMiscible9.1 cc/100 ccInsolubleMolecular Weight4674—Molecular FormulaC2H5OHC4H9OHC4~C12Density @20° C., g/cm30.790.810.72Boiling Point, ° C.78117 32~210Flash Point, ° C.1235−20Heat Value, Kcal/L5,0756,4047,700Evaporation Heat, Kcal/kg20014286Sensible Heat, Kcal/kg ° C.0.620.560.5Air to Fuel Ratio9.011.214.6Blending Octane Number12090~10091~99MON (Motor Octane Number)967881~89Vapor Pressure @100 F., psi20.33—
Despite the above advantages, the reason why biobutanol is not used as fuel is the high production price.
Because butanol is more toxic to organisms than ethanol, it cannot be accumulated to a sufficiently high concentration in a fermentation broth. In the case of typical ABE (Acetone-Butanol-Ethanol) fermentation (FIG. 1) using Chlostridium acetobutyricum, productivity of ABE is very low to the level of 0.2 g/h-L, and the maximum concentration of butanol in the fermentation broth is no more than about 1.3%, and thus a fermentation reactor should have a larger capacity relative to a production amount. In particular, the quantity of energy necessary for separation and concentration of butanol from the broth is very large, and thus the production price of biobutanol is considerably higher compared to bioethanol. Also, the strain for producing butanol undesirably loses its butanol production function from a certain point of time due to the toxicity of butanol.
EEI (Energy Environment Inc.), USA, reported a more efficient two-step fermentation process composed of producing only butyric acid using Clostridium tyrobutylicum as a strain and then selectively producing only butanol using Clostridium acetobutylicum as a strain (U.S. Pat. No. 5,753,474) (FIG. 1). As such, the productivity may be increased to 6 g/h-L using a fermentation reactor in which the strain is immobilized on a fibrous bed, but the maximum concentration of butanol in the broth is no more than about 2%.
In the case when biobutanol contained in the broth is distilled and recovered, as shown in FIG. 2, separation of 1 l of butanol consumes 5,000 kcal or more of energy and is thus very non-economical in consideration of the combustion heat of butanol being 6,400 kcal/l. Also, EEI proposes gas stripping as a method of recovering butanol present at a low concentration in a fermentation broth. However, compared to typical distillation, gas stripping is unfavorable in terms of energy costs.
Compared to bioethanol, in order to generate economic benefits, biobutanol requires improvement of a butanol fermentation strain for increasing the concentration of butanol in the broth and development of a separation technique for greatly decreasing the separation cost of butanol from the broth at a low concentration.
To reduce the separation cost of butanol from the broth, there is proposed a liquid-liquid extraction method, including recovering butanol from a fermentation broth using a specific solvent having a high butanol extraction coefficient and then recycling the solvent while recovering butanol using the difference in boiling point between the solvent and the butanol.
U.S. Pat. No. 4,260,836 discloses a liquid-liquid extraction method using a fermentation broth containing fluorocarbon having a high butanol extraction coefficient, and U.S. Pat. No. 4,628,116 discloses a liquid-liquid extraction method including liquid-liquid extraction of butanol and butyric acid from a fermentation broth using a vinyl bromide solution.
In Situ Extractive Fermentation of Acetone and Butanol (Biotech. and Bioeng., Vol. 31, P. 135-143, 1988) discloses a liquid-liquid extraction method of butanol using oleyl alcohol.
However, these liquid-liquid extraction methods are not commercially used This is considered to be because solvent extraction of low concentration biobutanol contained in the fermentation broth is still non-efficient.
Because the concentration of butanol in the fermentation broth is very low (less than 2%), even when the above extraction method is employed, there still occur problems in which the cost for separation and purification is excessively high and the strain for producing butanol loses a butanol production function. In order to solve these problems, the strain for producing butanol needs to be improved, but it is difficult to expect drastic improvement of the strain within a short time in consideration of the strain development rate to date.
In accordance with Extractive Fermentation for Butyric Acid Production from Glucose by Clostridium tyrobutylicum (Biotech. and Bioeng., Vol. 82, No. 1, P. 93-102, April 2003), as shown in FIG. 3, a fermentation broth discharged from a fibrous bed reactor is transferred to a hollow fiber membrane extraction column As such, in the extraction column, trialkylamine insoluble in water, for example, Alamine 336 is used as an extractant, and butyric acid is combined with trialkylamine and is thus converted into trialkylammonium butyrate and extracted. This extraction process is referred to as reactive extraction. Then, trialkylammonium butyrate is transferred to another hollow fiber membrane extraction column using sodium hydroxide as an extractant. In this extraction column, trialkylamine is recycled, and an aqueous sodium butyrate solution having a high concentration is obtained. When hydrochloric acid is added to the aqueous sodium butyrate solution, an aqueous butyric acid solution may be obtained. This process produces highly pure butyric acid but undesirably consumes 1 mol caustic soda and 1 mol hydrochloric acid to produce 1 mol butyric acid.
U.S. Pat. No. 4,405,717 discloses recovery of acetic acid including treating calcium acetate contained in a fermentation broth with trialkylamine carbonate to thus produce trialkylammonium acetate and calcium carbonate, concentrating the trialkylammonium acetate, and heating the trialkylammonium acetate thus obtaining acetic acid and trialkylamine.
Also, hydrogenation for converting carboxylic acid into corresponding alcohol using a chemical reaction is well known in petrochemical fields.